Instantaneous Heat Transfer Coefficient of Droplets Impinged on Heated Surfaces

Abstract

Droplet impingement is one of the phase change techniques used for equipment cooling because of the proper utilization of sensible and latent heat. This study analyzes droplet evaporation on aluminum and copper substrate. The instantaneous heat transfer coefficient (HTC) during evaporation of 100 µl de-ionized water is studied. The infrared camera is used to visualize and analyze the evaporation mechanism. The effect of substrate and fluid temperature on droplet evaporation is analyzed. The surface temperatures of both substrates are varied from 105 °C to 165 °C. The test fluid is maintained at temperatures of 30 °C, 50 °C, 75 °C, and 99.4 °C. The droplets exhibited Leidenfrost behavior for temperatures beyond 145 °C with copper and 150 °C with aluminum surface. It is observed that the evaporation rate is higher for copper than aluminum. A user-friendly interface is developed for determining the instantaneous surface area of an evaporating droplet. The apparent peak in the area in copper is higher than aluminum for all cases due to surface tension gradient and Marangoni flow. HTC increases with the temperature of droplet evaporating on a given substrate and is higher for copper. When substrate temperature increases for a given droplet temperature, the instantaneous HTC increases.